Protein in the urine of healthy dogs and cats is present in trace amounts. It can occur in larger quantities in the urine of sick animals, as well as during certain physiological conditions. Routinely, proteinuria is assessed using the urine protein to creatinine ratio (UPC), which is a widely recognized prognostic indicator. Under normal conditions, its value is <0.2. Values ranging from 0.2 to 0.4 in cats and from 0.2 to 0.5 in dogs are considered upper limit of normal, whereas UPC values >0.4 in cats and >0.5 in dogs are indicative of proteinuria. When diagnosing the causes of proteinuria in dogs and cats, it is important to consider damage to the glomeruli, renal tubules, interstitial tissue, and progressive loss of nephrons. Protein fractions present in urine, which vary in molecular weight, are sensitive diagnostic indicators of kidney dysfunction. Increased excretion of high molecular weight proteins may indicate damage to the renal glomeruli, while loss of low molecular weight proteins may suggest dysfunction of the renal tubules. If proteinuria persists and/or does not resolve after treating the underlying disease, it is necessary to initiate treatment aimed at reducing proteinuria by >90% in cats and >50% in dogs. The most effective treatment method for reducing proteinuria, while also having nephroprotective and cardioprotective benefits, is the blockade of the renin-angiotensin-aldosterone system (at its different levels). It is also essential to implement a proper diet, particularly by restricting protein and sodium intake.